Standing in a forward flexed position is an awkward posture due to the flexion extension moment created by the trunk as it bends forward away from the centerline. To hold this position the low back and hip extensor muscles must work continuously to counterbalance the moment composed of the weight of the upper body and external load, when applicable, times the length of the lever arm. Since the back extensors (erector spinae) work on a small lever arm an amplified tensile force is needed and as a result applies large compressive and shear forces through the lumbar spine. Stationary loading has been shown to increase the incidence of low back pain (Silverstein, Silverstein and Franklin, 1996), therefore lightening the biomechanical load on the lumbar spine should reduce the incidence of low back injuries. Once an injury has occurred, workers may be delayed in their return to work as they wait for sufficient healing and strengthening to take place so as their physical capacity matches the work demands.
Standing at work is a prevalent work posture (Tissot et al. 2005) but to date there are no effective assistive devices that lower the workload of the trunk muscles and reduce the compressive forces through the spinal joints (Swie and Sakamoto, 2004). In sitting, chair backrests have been shown to lower the muscular electromyography activity of the back extensors and abdominals (Makhsous, 2003) but this is not helpful when an individual leans forward to carryout a work task in front. An on-body personal lifting assist device (PLAD) has been shown to offset the spinal loads and reduces the electromyography of the erector spinae by 14%-21% and the compression and shear forces at L4/L5 by 13% to 15% (Abdoli et al. 2007, Lotz et al. 2007, Graham et al. 2008, Frost et al. 2008). PLAD is modeled on the concept of human muscle through the use of an elastic element that acts as an external muscle force generator but it has certain limitations associated with being an on-body device and appears best suited for dynamic work such as manual handling tasks with large vertical lifting. Alternate work postures include sitting or standing upright in a neutral position. Standing compared to sitting produces lower compressive forces through the lumbar spine (Callaghan & McGill, 2001), Psychophysically, however, the preference and perceived effort is mixed. Yates and Karwowski, (1992) found subjects perceived sitting to be harder and the authors attributed the difference to change in lumbar curvature. Kim et al., (2004) also found a higher perceived load in sitting but this was only for smaller subjects. Johnson and Nussbaum (2003), found that perceived effort was higher in a standing waist bend posture possibly due to more stability gained from leaning onto the fixture. An early study by Aaras et al. (1988) also showed that a seated posture was usually preferred despite increased load on the shoulder muscles and attributed to improved precision, stability, increased mobility and less load on legs and feet, less energy expenditure. Standing upright with the flexion extension moment neutral is ideal, however, this is not always possible and a forward flexed posture is needed when a work surface is below the elbow height or extreme reach beyond the length of the extended arm is needed. These two factors are unique to individuals and cannot always be accommodated by traditional ergonomic strategies.
Low back pain is the most common musculoskeletal complaint of workers with annual costs estimated at $12 Billion in 2002 in Canada (WorksafeBC, 2003) and $90 Billion in 1998 in the United States (Luo et al. 2004). Standing work has been identified as a risk factor for lower back pain as a result of the cumulative spinal loading and physiological work demands that result from the flexion moment that is created when the trunk leans forward from the upright neutral position. With as little as 10 degrees of forward lean the compressive loads through the lower lumber discs doubles (Takahashi et al., 2006). A recent population survey in Quebec showed that 58% of workers reported standing at work of which less than less 20% report that they can alternate position by sitting or walking (Tissot et al., 2005). Keyserling (1992), reported 89% of 335 surveyed manufacturing and warehouse jobs involved mild trunk flexion of less than 20 degrees. The degree of forward inclination depends upon individual anthropometry, design of workstation and nature of the work. The amount of forward leaning can be minimized by mechanical strategies that adjust workstation to fit the individual but this is not always practical, safe and can be costly.
An ergonomic device that safely lowers compressive loads may be effective in addressing the above-noted problems.